Polydiorganosiloxane-polycarbonate block copolymers are generally known. Typically, these copolymers are prepared either from a two-phase boundary process or a solvent-free melt process by reacting hydroxyaryloxy-terminated siloxanes with bisphenolic compounds or the oligomers derived therefrom.
Illustratively, U.S. Pat. No. 4,732,949 describes a two-phase boundary process for the production of a polydiorganosiloxane-polycarbonate block copolymer. The process includes reacting (a) a linear hydroxyaryloxy-terminated siloxane having the general structure (I)
(b) a bisphenolic compound, (c) a chain stopper, such as p-dodecylphenol, and (d) a chain-branching agent. A specific example of the disclosed bisphenolic compound is 2,2-bis(4-hydroxyphenyl)propane (II), wherein Y1-Y4 are all hydrogen.

U.S. Pat. No. 5,112,925 describes a two-phase boundary process for preparing a polydiorganosiloxane-polycarbonate block copolymer by reacting (a) a diphenol and (b) a hydroxyaryloxy-terminated siloxane wherein the aryl moiety in the terminal portion of the siloxane is a divalent radical of the general formula (III) or formula (IV):

According to the patentees, the copolymers derived from the siloxanes having the above described aryl moiety have improved surface tension as compared to the copolymers derived the siloxanes that do not contain those specific aryl moieties.
U.S. Pat. No. 5,783,651 describes a solvent-free melt process for preparing polydiorganosiloxane-polycarbonate block copolymers from (a) hydroxyaryloxy-terminated siloxanes of the general structure (I), (b) bisphenolic compounds, and (c) carbonic acid diaryl esters using specific quaternary ammonium or quaternary phosphonium compounds as catalysts.
U.S. Pat. No. 6,066,700 describes a solvent-free melt process for preparing polydiorganosiloxane-polycarbonate block copolymers from hydroxyaryloxy-terminated siloxanes of the general structure (V) and an oligocarbonate of the general structure (VI):

The oligocarbonates of formula (VI) are obtained from bisphenolic compounds and the diaryl carbonates (VII), whereby Ar=C6H5 is preferred.

There are three general paths to linear hydroxyaryloxy-terminated siloxanes of the general structure (I) known in the prior art: (A) U.S. Pat. No. 3,189,662 describes the reaction of chloroterminated polysiloxanes with bisphenolic compounds eliminating hydrochloric acid as the byproduct; (B) U.S. Pat. No. 4,732,949 describes the reaction of bisphenolic compounds with α,ω-bisacyloxypolydiorganosiloxanes in a solvent, and (C) U.S. Pat. No. 6,258,968 describes the reaction of bisphenolic compounds such as hydroquinone with a cyclic dialkyl siloxane such as octamethylcyclotetrasiloxane in a solvent, whereby an acid catalyst is used and water is removed from the reaction mixture by distillation.
Unfortunately, all these paths to linear hydroxyaryloxy-terminated siloxanes suffer from the disadvantage that both hydroxy functions of the bisphenolic reactant are equally likely to react with the siloxane substrate to produce, along with the desired hydroxyaryloxy-terminated siloxane, also byproducts, for example, polymers containing groups of the general structure (VIII):

These byproducts, if formed, represent undesirable impurities if incorporated into the polysiloxane-polyorgano block copolymers.
In addition, hydroxyaryloxy-terminated siloxanes prepared by the prior art processes tend to have batch to batch variations in terms of chain length because it is difficult to control the number of oligomer repeat units “m” shown in structure IX below in these siloxanes.

Such a variation in the chain length of polysiloxane would lead to structure variations of the resulting polysiloxane-polyorgano block copolymer, which in turn may effect the properties of the copolymer.
There are methods to control the number of oligomer repeat units “m”. For example, U.S. Pat. No. 4,732,949 describes using high dilutions of solvent and excess bisphenolic compounds to control the number of oligomer repeat units “m”. However, such a process is not cost-effective.
As an alternative to hydroxyaryloxy-terminated siloxanes, carbonate-terminated siloxanes have also been used to prepare polysiloxane-polycarbonate block copolymers. For example, U.S. Pat. No. 5,504,177 describes a solvent-free melt process for preparing polydiorganosiloxane-polycarbonate block copolymers using Si—O—C-free carbonate-terminated siloxanes of the general formula (X):

However, this path has the disadvantage that it requires the use of expensive p-allylphenolic precursors for the group R3 and the use of platinum catalysts, which both add to the cost of the process.
Accordingly, there is a need for a cost-effective process to produce polysiloxane-polyorgano block copolymers having a controlled siloxane block structure, yet at the same time avoiding the formation of undesired byproducts. The present invention provides an answer to that need.